iPhone is not the safest of the devices but certainly one of the most used devices. In the modern era smartphones are also used as a mobile data backup. As these phone can perform almost like a computer, many of the important folders are saved in iPhone. As research suggest iPhone is one of the basic sources of data loss. How can that data can be secured? Any suggestions?

apple built in encryption in iOS 6 and possibly 5. you can turn it on and your data will be encrypted but you'll have to enter a password to get to it. note there may or may not be a backdoor to this for government (Google police apple unlock) so you may want to keep your super secret stuff elsewhere and not on your phone. android has dmcrypt which is straight out of Linux and very secure. apple iOS I believe uses aes256 so it should be secure as well if you pick a good password

I have doubt that any mobile encrypting software uses aes256. Well, iPhone is one of the biggest source of losing data

Back up the documents to your PC in a locked folder? Take regular backups?

Making regular back up is not a remedy to avoid data breach

No, but it will guarranty you won't lose your data.